There will always be exciting new talent, and always be new fans who fall in love with MMA to keep it alive. There may not be another McGregor but the fans guys like Conor bring in either stick around too and become fans of the sport or they only care about that one fighter and were never long term anyways. It might under go a rebranding, rules might change over the next decade but it will still be here even when all the original fans are all senile and complaining about it twice as much as they already do.
And no, it's heavyweight, you don't exactly have to hit as hard a "Ford Focus" or whatever to be able to knock the other guy out + The older guys who still have their chins might tool Ngannou. Werdum with a gameplan could probably break him down, he might gas pretty hard in a 5 round fight.